Cellular telecommunications technologies continue to evolve to satisfy consumer demand for fast and reliable mobile communications. First generation (1G) analog communications systems have been superseded by second generation (2G) digital communications systems, such as the Global System for Mobile Communications (GSM) system. In the last several years these 2G systems have been enhanced by the introduction of General Packet Radio Service (GPRS) and Enhanced Data Rates for GSM Evolution (EDGE) wireless services (often referred to as 2.5 G and 2.75 G systems), which provide users not only with voice communication capabilities but also data communication capabilities. Currently, a third generation (3G) system known as the Universal Mobile Telecommunications System (UMTS), which employs the Wide-Band Code Division Multiple Access (W-CDMA) wireless service, is being implemented in many parts of the World, to provide even faster and more reliable voice and data communications.
While advances in cellular telecommunications systems have benefited consumers, the realization of higher data throughput and the increasing need for efficient use of available radio frequency (RF) spectra has led to more stringent telecommunications standards. These more stringent standards require handset manufacturers to provide solutions that operate according to more complex modulation schemes and enhanced power control conditions. For example, whereas GSM uses a constant envelope modulation scheme, EDGE and W-CDMA technologies employ more sophisticated non-constant envelope signals. EDGE and W-CDMA also require that the RF transmitter of a mobile terminal to control its output power over a wide dynamic range. Specifically, the EDGE standard requires a transmitter to have the ability of controlling output power over a 30 dB range, while the W-CDMA standard requires a transmitter to have the ability of controlling output power over an 80 dB range.
The wide dynamic range in output power control specified by the W-CDMA standard results from the fact that the W-CDMA wireless interface utilizes the direct sequence CDMA (Code Division Multiple Access) signaling method. All mobile terminals share the same radio resource in CDMA-based systems. Consequently, it is important that each physical channel between a base station and a mobile terminal not use more power than necessary. To accomplish this level of power control, W-CDMA systems use a transmit power control (TPC) mechanism, in which base stations in the network transmit TPC commands to mobile terminals in a downlink (DL) direction. The TPC commands require the mobile terminals to increase or decrease their transmission power levels in the uplink (UL) direction in increments (e.g., +/−1, 2, 3, etc. decibel (dB) increments), so that system power use is managed and maintained at acceptable levels.
Wide dynamic range in output power is difficult to achieve in conventional RF transmitters (e.g., those based upon quadrature modulators). The difficulty derives from the requirement that the power amplifier (PA) used in such transmitters operate with high linearity, so as to prevent, for example, spectral re-growth and unwanted adjacent channel interferers. The linearity requirement becomes especially problematic when non-constant envelope signaling schemes, such as EDGE and W-CDMA are used, since the drive levels to the PA must be reduced to avoid signal distortion caused by clipping of signal peaks. Additional linearization resources must also be provided to ensure signal integrity. Unfortunately, the immediate consequence of these efforts to preserve linearity is an overall reduction in power efficiency.
A polar modulation transmitter is an alternative approach that avoids the problems associated with the conventional quadrature-modulator-based transmitter. As explained below, the polar modulation transmitter is energy efficient, since the PA is not required to operate with high linearity, and is capable of controlling output power over a wide dynamic range.
FIG. 1 is an architectural diagram of a typical polar modulation transmitter 100. As shown, the polar modulation transmitter 100 comprises a polar signal generation circuit 102, an amplitude control circuit 104, a PA 106, an antenna 108, and a phase modulated signal generation circuit 110. In operation, the polar signal generation circuit 102 operates on an input signal to provide an envelope component signal containing amplitude information of the input signal and a constant-amplitude phase component signal containing phase information of the input signal. The envelope component signal is coupled to an input of the amplitude control circuit 104 along an amplitude path, and the constant-amplitude phase component signal is coupled to an input of the phase modulated signal generation circuit 110. The phase modulated signal generation circuit 110 is configured to receive the constant-amplitude phase component signal and generate a constant-amplitude phase-modulated RF drive signal, which is coupled to an RF input of the PA 106 along a phase path. The amplitude control circuit 104 is configured to receive the envelope component signal along the amplitude path and provide an amplitude modulated power supply voltage having a power level determined by a transmit power control signal coupled to a power control input of the amplitude control circuit 104. The amplitude modulated power supply voltage is coupled to a power supply port of the PA 106, which amplifies the constant-amplitude phase-modulated RF drive signal in the phase path according to the amplitude modulated power supply voltage, thereby providing a modulated RF output signal that is radiated by the antenna 108 to a system base station.
Because signal envelope variation and output power control in the polar modulation transmitter 100 are performed by varying the gain of the polar output stage 106, there is no need for RF circuit linearity, as there is in conventional transmitters. Both in-band and out-of-band output noise are also dramatically lower compared to output noise produced by conventional transmitters. Another benefit of the polar modulation transmitter 100 is that it is capable of controlling output power over a wide dynamic range. This is achieved by configuring the PA 106 to operate in compressed mode during times when a high transmission power is required, and switching to uncompressed mode during times when only a low transmission power is required. When configured in compressed mode the output power of the transmitter is controlled by the amplitude modulated power supply voltage applied to the collector (or drain) node of the PA 106, while the power of the constant-amplitude phase-modulated RF drive signal is kept constant. When configured in uncompressed mode, the output power of the PA 106 is controlled by varying the power of the phase-modulated RF drive signal, while the collector (or drain) node of the PA 106 is held constant.
Although the polar modulation transmitter 100 is fully capable of achieving a wide dynamic range in output power, even for W-CDMA applications where an 80 dB output power control range is required, drift in output power can make power control difficult. The Third Generation Partnership Project (3GPP), which is the standards body responsible for promulgating UMTS and W-CDMA standards, requires that TPC commands from a cellular network base station result in a mobile terminal increasing or decreasing its output power level in discrete steps (e.g., +/−1 dB, +/−2 dB, +/−3 dB, etc.). The UMTS standard also specifies that these power level steps be performed within certain specified tolerances. For example, as shown in the table in FIG. 2, a TPC command for a +/−1 dB step in output power level requires that the resulting output power be within +/−0.5 dB of the target output power. So, if a transmitter of a mobile terminal is operating at 0 dBm, and a TPC command of “1” is received, the transmitter of the mobile terminal must adjust its power so that it transmits within a range of +0.5 dBm and 1.5 dBm. Wider tolerances of +/−1 dB and +/−1.5 dB are permitted for larger step sizes of 2 and 3 dB. The 3GPP UMTS standard also imposes cumulative tolerances for groups of power commands, as shown in the table in FIG. 3. For example, ten equal TPC command groups of 1 dB step size each, requires that the resulting output power level be within +/−2 dB of the target output power level.
Inspection of the table in FIG. 2 reveals that the most restrictive step size tolerance for a single TPC command is for a TPC command directing a +/−1 dB step size (+/−0.5 dB tolerance is required). Unfortunately, this tolerance is not always easily satisfied by the polar modulation transmitter 100 in FIG. 1, during times when the power level step involves a mode switch from uncompressed mode to compressed mode. Ideally, the output power level following a mode switch from uncompressed mode to compressed mode is continuous. When the PA 106 is configured to operate in compressed mode, steps in power levels remain fairly accurate. However, as illustrated in FIG. 4, a discontinuity (or “gap”) is observed in the output power curve near the region where a mode switch occurs. This discontinuity is caused by drift in operating characteristics of circuitry within the phase path of the transmitter and in the power amplifier, which can be sensitive to temperature, aging, load conditions, and voltage variations. It has been observed that in some circumstances the discontinuity between the compressed and uncompressed mode power level curves is large enough that the +/−0.5 dB tolerance for 1 dB power step size specified by the UMTS standard is not satisfied. It would be desirable, therefore, to have methods and apparatuses for aligning the power level curves for step sizes involving a mode switch in a polar modulation transmitter, so that the UMTS power control accuracy requirements are satisfied.